The present invention relates to a process control apparatus capable of determining optimum control constants according to the characteristic of a process, which is obtained by using a sampled process value under closed loop.
In a process controller, since the control constants must be determined based on the dynamic characteristic of the process, identification of the dynamic characteristic of the process is important. Identification of the dynamic characteristic is performed either in an open loop wherein the controller is disconnected from the process or in a closed loop wherein the controller is connected to the process. From the viewpoints of economy, quality control and safety, it is preferable to perform identification in a closed loop which is able to response to the dynamic characteristic change of the process in operation.
A process controller for controlling the process on the basis of the dynamic characteristic obtained in a closed loop is described, for example, in K. J. .ANG.strom and B. Wittenmark, "On Self Tuning Regulators" in "Automatica", Pergamon Press, Great Britain, Vol. 9, pp. 185-199. As this controller is of minimum variance type, when the reference value changes, an large operation signal is generated. This is not preferable in view of the energy required for generating such an operation signal. The PID controller has been used widely in industry. However, there is not a conventional PID digital controller of which the PID control constants can be automatically determined from the dynamic characteristic of the process under closed loop operation. For this reason, with the conventional PID controller, the operator manually adjusts the control constants while closely observing the response of the process value. This results in long adjustment time and troublesome operation.